1. Field of the Invention
The present invention relates to an organic electroluminescent element.
2. Description of the Related Art
Electroluminescent elements comprise at least a light-emitting material. Organic electroluminescent elements are elements constructed using organic compounds which emit light in response to electric signals.
Organic electroluminescent elements basically comprise an organic luminescent layer interposed between a pair of opposed electrodes. Luminescence is a phenomenon which occurs when a luminescent body within the luminescent layer is excited to a higher energy level and the excited luminescent body returns to its original base state discharging said surplus energy as light. This is achieved by the injection of electrons from one electrode and the injection of holes from the other electrode.
In order to improve luminescence efficiency, the aforesaid basic construction is supplemented by a hole-injection layer added to the electrode that injects holes, while an electron transport layer is added to the electrode that injects electrons.
An example of an electroluminescent element is disclosed in U.S. Pat. No. 3,530,325 which uses single crystals of anthracene in the luminescent element.
Japanese Laid-Open Patent No. 59-194393 discloses a combination of a hole-injection layer and an organic luminescent layer.
Japanese Laid-Open Patent No. 63-295695 discloses a combination of an organic hole-injection/transporting layer and an organic electron-injection/transporting layer.
Organic electroluminescent elements having the aforesaid laminate layer construction are prepared by superimposing an organic fluorescent body, charge-transporting organic material (charge-transporting member), and electrodes, such that luminescence is generated when holes and electrons injected by their respective electrodes move within the charge-transporting member and recombine. Examples of materials which are useful as organic fluorescent bodies include organic pigments which generate fluorescence such as 8-quinolinol-aluminum complex, coumarin compounds and the like. Examples of useful charge-transporting materials include N'-di(m-tolyl)N,N'-diphenylbenzidene, 1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane and diamino compounds, 4-(N,N-diphenyl)aminobenzaldehyde-N,N-diphenylhydrazone compounds and the like. The use of porphyrin compounds such as copper phthalocyanine has also been proposed.
Although organic electroluminescent elements have high luminescence characteristics, prior art devices have been shown to have inadequate stability when luminescent and/or during storage. Thus, they are not suitable for practical use. The stability of the charge-transporting layer in prior art devices has been poor, resulting in instability in the element during luminescence and/or storage. The layers of the electroluminescent element formed by organic material range in thickness from a hundred to several hundred nanometers, and require high voltage application per unit thickness. Furthermore, such elements generate heat due to current flow and luminescence; therefore, the charge transporting material must be electrically, thermally, and chemically stable.